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Grindstones and Emery  Wheels.                195

Whitworth Guide Screwing Stock in Fig. 209. Here there
are three dies, a being the * guide,' cut so that its ridges just fit
the bolt at first, and are made to mark out the correct angle for
the top of the thread, b and c are the cutting dies (gradually
advanced by the wedge bolt d\ and these ultimately give the
correct form for the bottom of the thread. But the only perfectly
true method of cutting a screw is by means of the lathe, where
the tool is fixed in the slide rest and the thread formed by the
gradual advance of the rest coupled with the rotative movement
of the work. (See Appendix //,, p. 815,)

Machinists' requirements, in addition to the tools men-
tioned in Chapter V. These consist principally of grinding and
sharpening tools.

The Grindstone, though banished from some shops in
favour of emery, is still so extensively used as to deserve mention.
It is shewn at Fig. 210, and the stone fits on a square spindle
having journals at the ends, lying in simple bearings. Large
washers are placed on each face of the stone and the nut a
tightens these. Fast and loose pullies are provided for driving
by power, and a shield b to prevent the water flying about, the
latter being a necessary lubricant, c is a rest for the work, placed
rather high up, and as close to the stone as possible, to avoid
accidents. The direction of rotation of the stone is shewn by the
arrow, and the speed is such as to give from 800 to 1000 ft. per
minute surface velocity. It is not advisable to actually run the
machine in water as this tends to soften the stone.

The Emery Grinder is seen at Fig. 211. Its bearings are
longer than those of the grindstone, and its peripheral velocity
much higher, being about 5000 ft. per minute. A plentiful
supply of water is required for tool sharpening, otherwise, with
most emery wheels, the temper would be drawn and the wheel
become glazedf The water is shewn in the figure as coming from
a vessel above the wheel, but is sometimes supplied under pres-
sure from a small pump. Glazing is caused by the cementing
material becoming softened by the heat produced in grinding,
though properly the cement should wear gradually and fall away
with the emery powder, A very useful form of emery grinder is
shewn in Fig. 212, suitable both for tool grinding on the larger